An optical transmission signal fades with distance when traveling through any type of optical fiber telecommunication system and, thus, needs amplification. In this regard, optical fiber amplifiers are used to transform a weak input optical transmission signal into a strong output optical transmission signal. Optical fiber amplifiers contain optical fibers with cores doped with certain rare earth elements, such as, erbium, that amplify light at certain wavelengths. The amplified wavelengths depend primarily on the rare earth dopant and on the fiber composition. Typically, a rare earth doped optical fiber amplifier utilizes a light source from an external laser, such as a semiconductor pump laser, to excite the dopant atoms in the optical fiber from a ground state to a higher energy level, whereby light from an optical transmission signal having a signal wavelength can stimulate these excited atoms to emit their excess energy as light at the signal wavelength, thus resulting in an amplified optical transmission signal. The degree of amplification depends on the excitation power input, as well as on the excitation wavelength. Standard erbium-doped fiber amplifiers amplify light having a wavelength in the range of about 1520 and 1610 nanometers and are usually pumped by commercially available semiconductor pump lasers that emit light at either 980 or 1480 nanometers. Typically, the 980 nanometer pump laser has an output power of about 165 milliwatts, whereas, the 1480 nanometer pump laser has an output power of about 140 milliwatts.
As communication distances are increased, it becomes necessary to increase the pump laser power to achieve a higher gain, which is the ratio of the output power to the input power in a rare earth doped optical fiber amplifier. The gain of a rare earth doped optical fiber amplifier depends on pump absorption, among other factors. Pump absorption, that is, the pump energy absorbed by the rare earth doped optical fiber amplifier is generally increased by increasing the pump power launched into the optical fiber amplifier. One factor that has limited an increase in the gain is the output power provided by commercially available pump lasers employed in the manufacture of rare earth doped optical fiber amplifiers. A problem with simply increasing the power of the pump laser has been that it decreases the lifetime of the pump laser significantly. This has led to the utilization of multiple pump lasers with rare earth doped optical fiber amplifiers. For instance, one scheme of increasing pump laser power has been to utilize a bidirectional pumping configuration, which involves the use of two pump lasers pumping in opposite directions, with each pump laser having a different wavelength, for example, a 980 nanometer pump laser at the input end of the rare earth doped optical fiber and a 1480 nanometer pump laser at the opposite output end of the rare earth doped optical fiber. The use of a pump laser at the input end of an optical fiber is known as "forward pumping" or "co-pumping", that is, pumping in the same direction as that of the optical transmission signal, and the use of a pump laser at the output end of an optical fiber is known as "backward pumping" or "counter-pumping", that is, pumping in the opposite direction from that of the optical transmission signal. Alternatively, four pump lasers, two at the input end and two at the output end of the optical fiber can be utilized to increase the pump power input into the optical fiber.
In designing an optical fiber amplifier, a factor that must be taken into consideration is the generation of background noise or amplified spontaneous emission, generally referred to as ASE. ASE is a result of excited dopant atoms spontaneously returning to the ground state, and emitting a photon. Such spontaneously emitted photons are multiplied (amplified) by the optical fiber amplifier, thus resulting in background noise. The background noise figure is also increased by pump light decay along the optical fiber. Moreover, ASE cannot be entirely suppressed by increasing the input pump power given that ASE increases linearly with the gain of the optical fiber amplifier. The lowest noise figure is almost always achieved when pump light propagates in the same direction as the signal. At a typical wavelength of about 1550 nanometer, the 980 nanometer pump laser is known to provide a lower noise background in the optical fiber amplifier than the 1480 nanometer pump laser, whereas, the 1480 nanometer pump laser is known to provide a higher power efficiency than the 980 nanometer pump laser, thus, making the 980 nanometer pump laser the preferred choice for performance, particularly in view of the low noise figure that can be attained.
In light of the foregoing, it is desirable to provide a rare earth doped optical fiber amplifier that can utilize more input pump laser power. Also, it is desirable to provide a method for coupling multiple pump lasers to a rare earth doped optical fiber amplifier in order to provide high output power. Also, it is desirable to provide a configuration for efficiently coupling more pump power into a rare earth doped optical fiber amplifier. Furthermore, it is desirable to minimize loss in the optical transmission signal that is to be amplified and to minimize reflections between the multiple pump lasers utilized.